As is well known in the prior art, especially in the field of semiconductor manufacturing but also in certain areas of optics or biotechnology, a homogenous distribution of liquids on an essentially plane substrate may be achieved by rotating (spinning)a substrate around an axis normal to the plane given by its surface. By applying a viscous liquid onto the surface during spinning centrifugal forces affect a distribution of the liquid radially outwards over the surface. Such “spinning” technique is used to disperse e.g. lacquer, resins, photo resist on semiconductor substrates. Moreover it is utilized in the production of optical data storage technology to provide an essentially homogeneous layer of resin, lacquer, adhesive etc. A special case is the production of all type of DVD formats which require the bonding of two half-disks.
A standard process for such distribution method is:    1) Dispensing a liquid on the substrate to be coated; eventually rotating it slowly during this step to achieve a advantageous initial spreading.    2) Spinning the disk at high speed (typically a few hundred rpm up to 12.000 rpm) to homogeneously distribute the liquid. The thickness of the layer depends on parameters such as viscosity, temperature, rotation speed and rotation time.
For substrates with a center hole the profile of the spin coated layer thickness shows a low-high trend from the inner radius towards the outer edge. This is due to the fact that there is no liquid material at/close to the center hole which could flow outwards. This lack of material causes the reduced thickness at small radii.
The variation of the thickness distribution therefore will not be reduced to a minimized level by standard spin coating process. In order to achieve an optimized coating condition, an extra treatment during the spin coating process is required.
Moreover, for substrates without a center hole it is difficult to achieve ramped thickness distributions, as would be of interest e.g. for photo resist coating thickness in mastering applications.
It is therefore desirable to have a method to influence the radial thickness distribution during the spinning process. The radial thickness dependence of a liquids thickness is dictated by the physics of the spinning process and cannot be avoided with radially constant viscosity of the liquid. The objective of the invention therefore is to provide a method for controlling the viscosity of the liquid to be distributed during spinning.